This invention is related to an improvement in commercial rubber reclaiming processes. Specifically, it is related to the devulcanizing or depolymerizing step called the digester process, more specifically to the improvements resulting from the use of a new reclaim oil.
The conventional rubber reclaiming process can be divided into three major parts, preparation, breakdown, and refining. The preparation steps are: sorting of the scrap rubber articles, cracking or grinding, sifting, magnetic separation, and in-process storage. Breakdown is also called devulcanization and depolymerization and is commonly accomplished by either the heater process, the digester process, the Reclaimator process, or the high pressure steam process. Refining comprises mixing the depolymerized rubber with reinforcing and processing agents and various refining (passing the rubber through mill rolls) and straining steps.
Typically in the digester process, ground scrap rubber is blended in definite proportions with water, reclaiming agents (oils and catalyst), then mixed in an autoclave under conditions of elevated temperature (e.g. 180.degree. C.) and pressure for a period lasting from 1-10 hours.
The terms "reclaiming agent" and reclaiming oil are often used interchangeably, most being composite byproducts not well defined in composition. Reclaiming catalysts, on the other hand, are chemical compounds of which only small amounts are needed during the depolymerization. The phenol alkyl sulfides and disulfides are examples of effective reclaiming catalysts used in the rubber reclaiming industry today.
Reclaiming oils have more than one function. They are used to make the vulcanized scrap rubber thermoplastic again by plasticizing it, and to swell the scrap in order to make the fabric reinforcement easily accessible to the digesting solution. The swelling function of the reclaim oils aids in the tearing of the rubber on the refiner mills. In fact, the effectiveness of an oil in reclaiming is said to depend primarily on its effectiveness in swelling the scrap rubber (see Rostler, K.S. & White, R.M., "Swelling of Rubber," Rubber Age, Feb. 1946, page 585-590). Good penetration into the rubber is also desirable.
Because of these several functions, more than one (most often two) oils are often used in combination. Usually, the oil which is used to swell the rubber is volatile and is lost from the rubber when pressure is reduced in the blow-down of the digester, and when the reclaim rubber stock is worked in sheet mills at elevated temperatures. Volatile for purposes of this application means having an initial boiling point less than 240.degree. C., and blow-down is the release of pressure from the digester at the end of the digestion process.
The less volatile oils are retained in the stock to plasticize it and render it sufficiently soft and pliable during and after working. Unfortunately the escape of the more volatile oils into the atmosphere creates air pollution. Some tumor promoting or carcinogen activity has been attributed to alkyl naphthalenes (see Schemltz et al., Anal. Chem. 48, 645, 1976). The aromatic oils separated from petroleum for use in reclaiming processes usually contain alkyl naphthalenes.
One answer to the problem of pollution by these lighter reclaim oils is condensation. A condenser may be placed in the vent line above the digester for the recovery of these oils. This can be combined with the steam distillation of the mixture in the digester after the batch is finished for the recovery of even more of the oil (see Le Beau, D.S., "Science and Technology of Reclaimed Rubber," Rubber Chemistry and Technology, Volume 40, page 217-237, 1967, and Belgian Patent No. 621,764).
The staining properties of reclaim rubber are affected by the reclaim process. Minimum staining is important because of the use of reclaim rubber in such products as white sidewall tires. Therefore, it is desirable to use a minimum staining reclaim oil in the process. Such an oil should have few (or non-mobile) color bodies.
The state of the art in rubber reclaiming oils is represented in the following patents: U.S. Pat. No. 3,116,258 (mixtures of alkylbenzenes and alkylnaphthalenes); U.S. Pat. No. 2,806,821 (AlCl.sub.3 -polymerized ethylene as a reclaim oil); U.S. Pat. No. 2,449,879 (blend of volatile solvent naphtha and coke oven light oil compounds). Defensive publication No. T949,007 teaches the use of high levels of volatile armoatics in reclaim oil for manufacturing solubilized reclaimed rubber. The following literature articles are also of interest: Slavova, S. et al, God. Vissh.- Khim. - Technol. Inst. Burgas, Bulgaria 1972 (pub. 1973), 9(9), 315-26, CA82:126317v (mixture of aromatic and naphthene hydrocarbons); and Andreeva, V. and Svetaeva, E., Kokskhim, (1), 40 (1970) (styrene-indene resin used).
Most of the reclaiming oils known to the art contain a substantial portion distilling below 240.degree. C. During the venting of the digester, processing, and storage, a significant portion of the volatile oil escapes, decreasing the weight of plasticizer in the rubber, causing air pollution (or requiring recovery), and releasing an unpleasant odor. A reclaim oil which performs the several functions described above but which does not have these disadvantages would be highly desirable. Air pollution or the cost of recovery equipment could be reduced, and plasticization of the reclaimed rubber would be improved.